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It might not be immediately obvious that this is the case. Let's record
it in the description of `GTimeVal` itself and also in
`g_time_val_from_iso8601`.
We also drop an incorrect statement in the documentation for
`g_time_val_from_iso8601` stating that years up to 3000 were supported;
this is also not true for the same reason.
Related: #1509